1. Field of the Invention
The present invention relates to a method for production of a semiconductor device.
2. Description of the Related Art
It has become difficult to secure a sufficient capacitor capacitance in a DRAM with miniaturization of semiconductor devises in progress. For securing a sufficient capacitor capacitance, it is necessary to increase the height of the capacitor, to increase a surface area by employing a crown structure, and so on.
Above all, there is a method in which the surface area of a storage node is increased by employing a COB structure with a capacitor placed on a top layer of a bit line to increase the height of the storage node. Here, for increasing the height of the storage node, it is necessary to form a hole of a high aspect ratio.
However, when a hole of a high aspect ratio is formed by conventional anisotropic etching, a geometrical abnormality called a bowing shape occurs. The bowing shape is a geometry in which the middle portion 12 of the hole expands and a hole diameter (b) of a middle portion 12 of the hole is larger than an entrance hole diameter (a) of an oxide film 3 just below a mask 4 when a hole section is circular as shown in FIG. 2. When the hole has a shape other than a circular shape, a sectional area Sb of the section of the middle portion 12 of the hole is larger than a sectional area Sa of an entrance hole of the mask (sectional area of an oxide film entrance just below the mask).
A process in which this bowing shape occurs when using a conventional production method is shown in FIGS. 3(A) to 3(F). First, a contact plug 1 as a primary layer, a nitride layer 2 as an etching stopper, an oxide film (interlayer insulating film) 3 and a polysilicon layer 4 as a hard mask are deposited one after another. Then, anisotropic etching is carried out by a lithography technique to pattern the polysilicon layer 4 (FIG. 3(A)). Next, the oxide film 3 is subjected to anisotropic etching using the patterned polysilicon layer 4 as a mask. At this time, etching proceeds mainly in a longitudinal direction 10. However, in a middle portion 11 of the oxide film, etching also proceeds in a lateral direction 12, and a geometrical abnormality called a bowing shape occurs (FIG. 3(B)). As a result, in the middle portion 11 of the hole, the distance between adjacent holes becomes very short, or the adjacent holes merge with each other.
The reason why the bowing shape thus occurs can be considered as follows. That is, when dry etching is carried out as anisotropic etching, charging occurs near the surface of a hole due to a difference in the amount of electrons and ions incident to the inside of a fine pattern when they are irradiated to the surface of a mask for forming a pattern in a plasma in which a gas is separated into electrons and ions. It can be considered that in this way, the course of incident ions is bent so that the ions impinge upon the middle portion 11 of the oxide film in the lateral direction 12. Furthermore, the ratio of incident ions being bent in the lateral direction is considerably influenced by the aspect ratio of the hole, and becomes noticeable when the aspect ratio exceeds 12. Thus, a bowing shape generally occurs when the aspect ratio exceeds 12.
Next, a wet etching treatment with a fluoric acid solution is carried out as a pretreatment for depositing a polysilicon layer. At this time, the distance between adjacent capacitors further decreases, and in the middle portion 11 of the hole, adjacent holes merge with each other (FIG. 3(C)).
Thereafter, a polysilicon layer 13 is deposited on the entire surface (FIG. 3(D)), and the polysilicon layer 4 and the polysilicon layer 13 deposited on the top surface and the side surface of the polysilicon layer 4 are then removed (FIG. 3(E)). Further, thereafter, the oxide film 3 is partly removed by carrying out wet etching to expose a part of a lower electrode (polysilicon) to a desired height (FIG. 3(F)).
When a bowing shape thus occurs, there have arisen the following problems.
(1) When a conductive film such as a polysilicon film as a lower electrode is formed after formation of a hole, the conductive film is stepped and broken at the location of a bowing shape.
(2) When the bowing shape occurs, adjacent holes merge with each other to cause a short circuit.
(3) Even if a short circuit does not occur, the distance between adjacent contact holes decreases so that a parasitic capacitance increases to deteriorate device characteristics.
(4) If a conductive film is buried in a hole where a bowing shape occurs, a void occurs at a central portion of the hole. If the void occurs, a gas in the void expands to cause bursting in a subsequent heating step.
So, various methods for preventing the occurrence of the bowing shape have been proposed. FIGS. 1(1) to 1(6) is a process sectional view showing a production method for inhibiting a bowing shape, which is proposed in Japanese Patent Laid-Open No. 2004-335526 (Patent Document 1) by the applicant.
In this production method, first, a silicon substrate 100 is prepared, and a silicon oxide film 101 and polysilicon 102 as a hard mask at the time of etching are formed one after another on the silicon substrate 100. Next, by a photolithography technique, a hole is provided in a photoresist (not shown) formed on polysilicon 102 and polysilicon 102 is dry-etched by the hole to form a hard mask for etching. Thereafter, the photoresist is removed (FIG. 1(1)).
A hole of an aspect ratio of 7 or more and 12 or less having a depth at which a bowing shape does not occur is formed in the silicon oxide film 101 by the first etching via the hard mask 102 (FIG. 1(2)). Next, a Si3N4 film 103 is deposited on the entire surface by a low pressure CVD method (FIG. 1(3)). Thereafter, the entire surface is dry-etched to remove the Si3N4 film 103 deposited on the top surface of polysilicon 102 and the Si3N4 film 103 deposited on the bottom of the hole. The Si3N4 film 103 is left only on the side surface of the hole (FIG. 1(4)).
At this time, it is necessary that the Si3N4 film 103 left on the side surface of the hole should cover at least a position of an aspect ratio of 2 to 7 in which a bowing shape is caused to occur in the hole. Thereafter, a hole extending to the silicon substrate 100 is formed (FIG. 1(5)) and the Si3N4 film 103 left on the side surface of the hole is removed (FIG. 1(6)) while carrying out dry etching as the second etching.
Owing to the production method of Japanese Patent Laid-Open No. 2004-335526, occurrence of a bowing shape was somewhat inhibited, but as shown in the enlarged view of the circled portion in the lower part of the hole in FIG. 1(6), the hole width in the lower part of the hole was narrowed in some cases. In addition, with further miniaturization of wiring rules, the hole diameter itself further decreased and the aspect ratio of the hole further increased, and the hole diameter in the lower part of the hole further decreased.
Particularly, when HSG (hemispherical grained) polycrystalline silicon was formed in the lower part of the hole, formation of the lower part of the hole as in FIG. 1(6) brought HSG polycrystalline silicon into mutual contact in the lower part of the hole so that a surface area of a design value could not be obtained in some cases. Here, when the diameter of HSG polycrystalline silicon was reduced for preventing the contact of HSG polycrystalline silicon, the surface area of a cylinder decreased in some cases. Therefore, prevention of a decrease in the hole diameter in the lower part of the hole has been a serious problem.
So, the present inventor conducted vigorous studies, and resultantly found that (1) occurrence of a bowing shape can be prevented at the time of the second etching by providing a mask layer on a portion in a hole where the bowing shape occurs after the first etching and (2) the hole diameter in the lower part of the hole can be increased by newly providing a step of enlarging the hole diameter in the lower part of the hole (side surface portion in the hole on which a mask layer is not provided) after the second etching.
That is, an object of the present invention is to provide a method for production of a semiconductor device having a hole extending through the first insulating film, the second insulating film and the third insulating film, wherein a bowing shape does not occur even at a high aspect ratio and the lower part of a hole has a large hole diameter.